Hematopoietic stem cells (HSCs) are clonogenic cells, which possess the properties of both self-renewal (expansion) and multilineage potential giving rise to all types of mature blood cells. HSCs are responsible for hematopoiesis and undergo proliferation and differentiation to produce mature blood cells of various lineages while still maintaining their capacity for self-renewal. The ability to self-renew maintains the HSC population for the lifespan of an animal and also allows HSCs to repopulate the bone marrow of lethally irradiated congenic hosts.
Early HSC development displays a hierarchical arrangement, starting from long-term (LT-) HSCs, which have extensive self-renewal capability, followed by the expansion state, which corresponds to short-term (ST-) HSCs (having limited self-renewal ability) and proliferative multipotent progenitors (MPPs) (having multipotent potential but no self-renewal capability). MPP is also a stage of priming or preparation for differentiation. An MPP differentiates and commits to become either a common lymphoid progenitor (CLP), which gives rise to all the lymphoid lineages, or a common myeloid progenitor (CMP), which produces all the myeloid lineages. During this process, the more primitive population gives rise to a less primitive population of cells, which is unable to give rise to a more primitive population of cells. The intrinsic genetic programs that control these processes including the multipotential, self-renewal, and activation (or transient amplification) of HSCs, and lineage commitment from MPP to CLP or CMP, remain largely unknown.
To sustain constant generation of blood cells for the lifetime of an individual, HSCs located in bone marrow niches (Zhang, J. et al. Nature 425, 836-841, 2003; Calvi, L. M. et al. Nature 425, 841-846, 2003; Kiel, M. J., et al. Cell 121, 1109-1121, 2005; Arai, F. et al. Cell 118, 149-161, 2004) must achieve a balance between quiescence and activation so that immediate demands for hematopoiesis are fulfilled, while long-term stem cell maintenance is also assured. In adults, homeostasis between the quiescent and activated states of stem cells is important to protect HSCs from losing their potential for self-renewal and, at the same time, support ongoing tissue regeneration (Li, L. and Xie, T. Annu. Rev. Cell. Dev. Biol. 21, 605-631, 2005). Over-activation and expansion of stem cells risks both eventual depletion of the stem cell population and a predisposition to tumorigenesis. Although some factors important for stem cell activation have been identified (Heissig, B. et al. Cell 109, 625-637, 2002), the molecular events governing the transition between quiescence and activation are poorly understood.
Phosphatase and tensin homolog (PTEN) functions as a negative regulator of the PI3K/Akt pathway, which plays crucial roles in cell proliferation, survival, differentiation, and migration (Stiles, B. et al. Dev. Biol. 273, 175-184, 2004). The PTEN tumor suppressor is commonly mutated in tumors, including those associated with lymphoid neoplasms, which feature deregulated hematopoiesis (Mutter, G. L. Am. J. Pathol. 158, 1895-1898, 2001; Suzuki, a. et al. Immunity 14, 523-534, 2001). PTEN-deficiency has been associated with expansion of neural and embryonic stem cell populations (Groszer, M. et al. Science 294, 2186-2189, 2001; Kimura, T. et al. Development 130, 1691-1700, 2003). But, the role of PTEN in stem cells and tumorigenesis and the recurrence of tumors heretofore has been not understood.
PTEN functions as an antagonist of phosphatidyl inositol 3-kinase (PI3K) (Maehama T & Dixon J E. J Biol. Chem. 273:13375-13378. 1998). The serine kinase Akt is downstream of the PI3K signal (Cross D A, Alessi D R, Cohen P et al. Nature 378:785-789 1995). PTEN has been shown to inhibit Akt and thereby inhibit the nuclear accumulation of β-catenin (Persad S et al. J Cell Biol. 153:1161-1174 2001).
Akt has a broad range of effects. Its major function is to provide a survival signal and to block apoptosis, complementary to its regulation of β-catenin function. (Song, G. et al., J. Cell. Mol. Med., 9 (1): 59-71, 2005) Akt acts through a number of proteins, including mammalian target of rapamycin (mTOR), the Forkhead family of transcription factors (FoxO), BAD, caspase 9, murine double minute 2 (Mdm2).
Akt can directly and indirectly activate the serine/threonine kinase mTOR, which activates protein translation through a signaling cascade. (LoPiccolo, J., et al., Anti-Cancer Drugs, 18:861-874, 2007). Indirect activation occurs through tuberous sclerosis complex-2 (TSC2), which, when in the unphosphorylated state, forms a complex with tuberous sclerosis complex-1 (TSC1, also known as hamartin). This complex promotes the GTPase activity of Ras homolog enriched in brain (RHEB), which in turn, acts to down-regulate mTOR activity. Upon phosphorylation by Akt, however, the ability of the TSC1-TSC2 complex to promote RHEB's GTPase activity is inhibited, and therefore, mTOR's activity is promoted. (Cully, M. et al., Nat. Rev. Cancer, 6:184-192, 2006). mTOR can also form a complex with Rictor, and this complex can provide positive feedback on the Akt signaling cascade by phosphorylating and activating Akt. (Sarbassov, D. D., et al., Science, 307: 1098-1101, 2005).
Akt also regulates cell survival through transcriptional factors, including FoxO. Akt's phosphorylation of FoxO inhibits FoxO, resulting in inhibition of transcription of several proapoptotic genes, such as Fas-L, IGFBP1 and Bim. (Datta, S. R., et al., Cell, 91:231-241, 1997; Nicholson, K. M., et al., Cell Signal, 14:381-395, 2002).
One of the down-stream targets of FoxO is p27 (Kip1), a potent inhibitor of cyclin E/cdk2 complexes. (Wu, H. et al., Oncogene, 22: 3113-3122, 2003). FoxO factors induce expression of p27, which can bind to cyclin E/cdk2 complexes and inhibit their activity, resulting in a block in cellular proliferation. (Burgering, B. M. T. & Medema, R. H., J. Leukocyte Biol., 73:689-701, 2003). In addition, Akt itself can also directly phosphorylate p27 on T157, resulting in the redistribution of p27 from the nucleus to the cytoplasm, away from cyclin E/cdk2 complexes. (Id.) Phosphorylation of p27 on T198 was critical for the binding of p27 to 14-3-3 proteins, and through this pathway, Akt may directly promote p27's degradation. (Fujita, N., et al., J. Biol. Chem., 277 (32): 28706-28713, 2002).
Another one of the targets of Akt in promoting cell survival is BAD, a member of the Bcl-2 family of proteins. In the absence of Akt phosphorylation, BAD forms a complex with Bcl-2 or Bcl-X on the mitochondrial membrane and inhibits the anti-apoptotic potential of Bcl-2 and Bcl-X. (Song, G. et al., J. Cell. Mol. Med., 9 (1): 59-71, 2005) Akt phosphorylates BAD on Serine 136, thus releasing BAD from the Bcl-2/Bcl-X complex. (Song, G. et al., J. Cell. Mol. Med., 9 (1): 59-71, 2005; Datta, S. R., et al., Genes Dev., 13:2905-2927, 1999). Therefore, Akt suppresses BAD-mediated apoptosis and promotes cell survival.
Furthermore, by phosphorylation of pro-caspase-9 at Serine 196, Akt inhibits proteolytic processing of pro-caspase-9 to the active form, caspase-9, which is an initiator and an effecter of apoptosis (Cardone et al., 1998, Science, 282: 1318-1320, Donepudi, M. & Gruner, M. G., Biophys. Chem., 145-152, 2002).
Additionally, Akt regulates cell survival via the Mdm2/p53 pathway. Akt can activate Mdm2 by direct phosphorylation, thereby inducing the nuclear import of Mdm2 or the up-regulation of Mdm2's ubiquitin ligase activity. (Mayo L. D., Donner D. B., 2001, Proc. Natl., Acad. Sci. USA 98:11598-11603; Gottlieb T. M. et al, Ocogene, 21: 1299-1303, 2002). Mdm2 negatively regulates the p53 protein, which may induce cell death in response to stresses (Oren M., Cell Death Differ., 10:431-442, 2003), by targeting p53 for ubiquitin-mediated proteolysis (Haupt, Y. et al., 1997, Nature 387: 296-299) or by binding to the transactivation domain of p53, thereby inhibiting p53-mediated gene regulation. (Momand, J. et al., Cell, 69: 1237-1245, 1992) One of the down-stream targets of p53 is the p21 (CIP1/WAF1) gene. The p53 gene product binds to a site located 2.4 kb upstream of the p21 coding sequence, and this binding site confers p53-dependent transcriptional regulation. (El-Deiry, W. S., et al., Cell, 75: 817-825, 1993) Thus, down-regulation of p53 also down-regulates the transcription of p21.
PTEN not only regulates p53 protein through antagonizing the Akt-Mdm2 pathway, it can also directly regulate p53. First, PTEN can enhance p53 transactivation in a phosphatase-independent manner (Tang, Y. & Eng C., Cancer Research, 66: 736-742, 2006). Second, PTEN forms a complex with p300 in the nucleus and plays a role in maintenance of high p53 acetylation, which is the activated form of p53. (Li A. et al., Molecular Cell, 23 (4): 575-587, 2006). In turn, p53 may also activate the transcription of PTEN. (Cully, M. et al., Nat. Rev. Cancer, 6:184-192, 2006).
Canonical signals in the Wnt pathway are involved in stem cell proliferation. (Kim, L. & Kimmel, A. R. Current Drug Targets 7:1411-1419, 2006). Glycogen synthase kinase 3 beta (GSK-3β) is a part of the Wnt signaling pathway, and its primary substrate is β-catenin. (Hagen, T et al., J. Biochem. 277 (26):23330-23335). In the absence of canonical Wnt signaling, GSK-3β binds to β-catenin and phosphorylates β-catenin, thereby targeting β-catenin for ubiquitination and followed by proteosome-mediated degradation, which is mediated by Adenomatous Polyposis Coli (APC). (Id., Moon, R. T. et al., Science 296:1644-1646. 2002). Canonical Wnt signals induce the release of β-catenin from GSK-3β, thereby activating β-catenin. (Katoh, M & Katoh, M. Cancer Biol Ther. 5 (9):1059-64, 2006). β-catenin then localizes to the nucleus, where it activates gene transcription. (Id.).
In sum, individually it was known or suggested that modulating GSK-3β and PTEN could promote stem cell, e.g., HSC, proliferation. But, such knowledge and speculation was insufficient to provide a clinically useful approach to expanding a stem cell population for subsequent transplantation and reconstitution of the engrafted cell lineage.
In view of the foregoing, it would be advantageous to elucidate the interaction between Wnt and PTEN signaling pathways and to provide new insights into molecular regulation of stem cell proliferation and differentiation. It would also be advantageous to use such insights to provide new methods and compositions for expanding stem cells in vivo and ex vivo, which stem cells would be of the kind and quantity sufficient to transplant into a suitable recipient.